This invention relates to a method and apparatus for controlling the injection step of an injection molding machine utilized for effecting precise molding.
To effect precise molding, the injection step must be controlled to provide precise filling and pressure maintenance or holding. More particularly; during the filling step, for the purpose of minimizing as far as possible the viscosity difference between the leading end and the trailing end of molten resin flowing into a metal mold cavity, it is necessary to fill the molten resin in a shortest time, while in the pressure holding step succeeding the filling step, it is necessary to adequately compensate for a shrinkage caused by the cooling of the molten resin filled into the metal mold cavity. Consequently, it is essential to effect switching in a short time from the filling step to the pressure holding step while maintaining a correct switching position and stability of repetition.
As a result of careful observation of the performance of the molten resin in the metal mold cavity, during the filling and pressure holding steps it has been noted that the degree of forming a skin layer is determined by the thermal balance as follows.
1. Heat supplied by the molten resin, heat generated by internal friction, and heat generated by compression.
2. Heat loss caused by heat conduction and thermal expansion.
Consequently, so long as the temperature of the molten resin and the temperature of the cavity surface are the same, larger is the flow speed, slower is the growth of the skin layer. Taking a polycarbonate resin as an example, the flow resistance is shown by the following equation. ##EQU1## where P: dynamic pressure loss
V: mean flow quantity PA1 L: flow distance PA1 H: height of cavity section.
As can be noted from this equation, when the flow speed decreases and the growth of the skin layer proceeds, the force necessary to cause flow becomes large with the result that unwanted stress would be applied to the skin layer. In this manner, the higher is the injection speed during the filling step, the slower is the speed of growth of the skin layer which improves the accuracy influenced by the internal stress, that is the dimensional accuracy, mechanical accuracy, physical strength, etc. However, there is a tendency that such defects as flow marks and silver streaks are formed on the surface of the product and that switching from the filling stroke to the pressure holding step becomes unstable. For avoiding these defects it has been the practice to adopt a programmed injection method in which the injection stroke is divided into a plurality of stages of different filling speeds.
Although according to this method it is possible to fill the molten resin by variously varying the filling speed, after switching to the pressure holding step, the control necessary to compensate for the shrinkage caused by the cooling of the molten resin filled in the mold cavity becomes difficult.
During the pressure holding step, the flow of the molten resin is substantially zero and the amount of heat loss is much larger than the quantity of heat generated, whereby the thickness of the skin layer increases rapidly. At this time, if the molten resin flows, stress is applied to the skin layer under growth, thus failing to ensure the accuracy.
I have made various investigations for finding out a method that can perfectly stop the axial movement of a screw or plunger (in the following description both are represented by a term plunger) that directly gives a flow to the molten resin during the pressure holding step and found the following. When melted, resinous materials are compressible liquids having a compressibility of 4-5% under a pressure of 600 Kg/cm.sup.2 and of 8-12% under a pressure of 1000 Kg/cm.sup.2, although different depending upon the type of the resins. For this reason, after the molten resin has been filled in the mold cavity under a precisely controlled pressure, satisfactory product can be obtained by rapidly removing external pressure, so as to compensate for the shrinkage of the product by releasing the pressure accumulated in the product. In other words, it is advantageous that the product is formed while dissipating the internal stress of the product. This invention is based on this discovery.